1. Field of the Invention.
The present invention relates to semiconductor devices, and more particularly, to aluminum contacts to metallic compounds in a semiconductor device.
Contact between a metal such as aluminum and a semiconductor generally results in two types of metal-semiconductor junctions. One type is a rectifying contact forming a metal-semiconductor diode (often referred to as a Schottky barrier or Schottky diode) having volt-ampere characteristics very similar to those of a p-n junction diode. The other type of contact, often referred to as an ohmic contact, is non-rectifying and is generally used when a lead is attached to a semiconductor.
2. Description of the Prior Art.
These ohmic and Schottky diode contacts can also be made between a semiconductor and metal-semiconductor compounds such as silicides which are compounds of silicon and a metal. The silicide is typically formed by depositing a metal layer such as nickel, cobalt, titanium, etc. onto a silicon substrate and subjecting it to a heat treatment known as annealing. During the annealing process, the metal layer spreads into the silicon substrate forming a silicide region in contact with the remaining silicon substrate. The silicide-silicon contact may be either an ohmic or Schottky diode contact depending upon the doping level of the silicon.
Silicide contacts to the silicon substrate are desirable because of the reproducibility of their electrical characteristics. However, aluminum frequently remains a preferred metal for the final contact to bonding pads or to other devices on the substrate. Aluminum contacts are typically formed by depositing a layer of aluminum on the region to be contacted (FIG. 1A). The aluminum layer is then generally treated to a heat treatment referred to as sintering. Sintering the aluminum layer allows the aluminum to form a better contact with the underlying region.
It has been found, however, that many aluminum-silicide contacts exhibit a thermal instability during the sintering cycle. That is, many silicides react with the aluminum in the temperature range used to sinter the aluminum contact (typically 400.degree.-500.degree. C.). This chemical reaction leads to the formation of intermetallic compounds from the aluminum and the silicide. Thus, for example, sintering an aluminum contact layer on a nickel silicide region has been found to form the intermetallic NiAl.sub.3 (FIG. 1B). The chemical reaction between the aluminum and the silicide further has been found to raise the Schottky barrier height of the silicide-silicon junction, which raises the forward voltage drop of the junction. This in turn increases the power consumed by the junction and may make the silicide contact unsuitable for many applications. In addition, the aluminum-silicide reaction may not proceed uniformly during the sintering cycle of the aluminum. As a result, the current flow through the sintered aluminum-silicide contact may not be uniform. This can lead to excessive current in some locations within the contact resulting in failure of the device.
In order to prevent the silicide from reacting with the aluminum during the sintering of the aluminum contact, it has been proposed (G. J. Van Gurp, J. C. C. Dames, A. Van Oostrom, L. J. M. Augustus, and Y. Tammings, J. Appl. Phys. 50, 6915 (1979)) to "sputter" deposit a 2,000 angstrom (0.2 micron) layer of tungsten on the silicide before depositing the aluminum to provide a barrier between the aluminum and the silicide. (Alternatively, a 1,000 angstrom layer of a tungsten-titanium composition was also proposed.) It was reported that the sputter deposited intermediate layer of tungsten (or tungsten-titanium) was effective in preventing a chemical reaction between the aluminum and the silicide. Because of the relatively large thicknesses of these intermediate layers (1,000-2,000 angstroms), it is generally required that these layers be sputter deposited with a sputtering apparatus. Other methods for depositing a metal layer, such as evaporation deposition with an electron beam gun, generally are not suitable for deposting layers of tungsten in excess of 250 to 500 angstroms thick.
Accordingly, Van Gurp, et al. suggests that after the annealing of the silicide region is complete, the device be removed from the annealing furnace and placed in a separate sputtering apparatus to have the tungsten layer sputter deposited on the silicide. It is then proposed that the aluminum layer be deposited on the tungsten layer by evaporation using an electron beam gun (E-gun). Because sputter depositing typically requires a separate apparatus, it would be highly desirable to eliminate the need for depositing the intermediate tungsten barrier layer on the silicide region by sputter depositing. This will allow devices using such a contact barrier layer to be manufactured more economically.